Production of porous shaped articles



United States Patent@ PRODUCTION F POROU SHAPED ARTICLES Leo Schlecht, Ludwigshafen (Rhine), and Herman Klippel, Hassloch, Germany, assignors to Badische Anilin- & Soda-Fabrik Alstiengesellschaft, Ludwigshafen, Rhineland-Pfalz (Rhine), Germany No Drawing. Application May 8, 1952, Serial No. 286,806

Claims priority, application Germany May 11, 1951 3 Claims. (Cl. 75-213) This invention-relates to a new andimproved process for the production of porous shaped articles.

It has already been proposed to manufacture highly porous shaped articles by sintering metal powder having a very low bulk density in molds. Thus it has been proposed to make highly porous electrode plates for accumulators by sintering nickel powder which has been obtained by thermal decomposition of nickel carbonyl and which has a bulk density of less than 2 kilograms per litre. In recent investigations, however, the drawback has been observed that the pore width of the porous shaped articles differs greatly. For example from a carbonyl nickel powder having a bulk density of about 1 kilogram per litre there are obtained porous electrode frames in which the largest pores are from to even 50 times as large as the smallest pores. There is consequently a considerable difference in the width of the individual pores. For many purposes, however, for example for the production of accumulator plates or filter plates, the most uniform pore width possible is essential.

We have now found that porous shaped articles having uniformly distributed pores of practically uniform width are obtained from powders by the so-called powdermetallurgical method, i. e. by sintering and/or a pressure treatment when powders are used of which the coarser particles which are agglomerated to form a sponge have been reduced by a mechanical treatment to smaller spongy particles. It has been found that light powders, for example metal powders having a very low bulk density, consist of spongy agglomerates which are so bulky that during the thermal and/or the pressure treatment large hollow spaces are left between the individual agglomerates which are responsible for the marked differences in the pore width of porous shaped articles.

If such agglomerates are subjected, before the pressure and/ or thermal treatment, to a mild mechanical treatment by which the coarse-grained agglomerates are broken up into fine-grained agglomerates, smaller spaces, as results from a microscopic investigation, are formed between these fine-grained agglomerates which also yield smaller pores by pressing and/or sintering. The coarse agglomerates may be most simply broken up into smaller agglomerates by passing the powder carefully through a sieve. For example the powder may be forced by means of a brush through a sieve having meshes of proper width and being made of wires which are advantageously pointed upwardly like wedges in order to ensure a better cutting action on the agglomerates to be broken up. Force should not be used during the sieving of the powder as in this way the agglomerates are not torn apart but crushed and the bulk density of the powder increased to an extent that the subsequent pressure and/or thermal treatment will no longer give a satisfactory porosity.

With certain powders the coarse agglomerates may also be broken up by magnetic separation, by wind sifting or by not too powerful shaking and in this way, too, no excessive increase in the bulk density will result.

The process according to our invention affords a special advantage in the production of porous sintered articles from light metal powders. The metal powders used are preferably obtained by reduction of very light metal oxides or more preferably by thermal decomposition of metal carbonyls. It is when a specially light powder is prepared by the thermal decomposition of metal carbonyls that coarse, cotton-Wool-like flocks are frequently formed in the form of agglomerates having a diameter of up to 1G millimetres or more from which highly porous shaped metal articles can be made by heating, but in which the width of the pores shows a marked variation. Such sintered articles often contain holes having a width up to 1 millimetre. According to our invention, however, shaped articles can be made from light carbonyl metal powders in which the width of the pores is practically of equal size and the pores themselves are uniformly distributed throughout the shaped article. This uniform distribution of the pores which are practically of the same size is especially important, for example, when a porous shaped article is to be used as a framework for an accumulator electrode because a uniform utilization of the active mass introduced into the pores and a small internal resistance spread uniformly over the whole electrode is thereby ensured.

Also in the case of filter plates the said uniform distribution of the pores of substantially the same size throughout the shaped article is of considerable advantage because the filtering action becomes more uniform and more certain.

Our invention also provides the possibility to regulate the pore width uniformly within certain limits by splitting up the abovementioned primary agglomerates into secondary agglomerates of different sizes. The pores obtained when the secondary agglomerates frit together and/or are compressed are the smaller the smaller the size of the said secondary agglomerates.

The best results are obtained by fritting together the secondary agglomerates by heating to sintering temperature. Pressure may also be employed before, during or after the heating. The breaking up of the primary agglomerates also result in advantages when the powders are subjected to a pressure treatment only.

The following example will further illustrate this invention but the invention is not limited to this example.

Example A nickel powder having a bulk density of 0.5 kilogram per litre obtained by thermal decomposition of nickel carbonyl contains cotton-wool-like agglomerates having a diameter up to about 1 centimetre. By heating this powder in a mold at 800 C., a porous plate is obtained the width of the pores of which varies between 10 and 300 If the same carbonyl nickel powder is carefully forced through a sieve having 400 meshes per square centimetre by means of a flexible scraper of synthetic material, a highly friable light powder is obtained; from this powder under the same sintering conditions there is obtained a porous plate having a pore volume of 89%. The pores of the plate are, however, only 10 to 25, in size and moreover are uniformly distribtued throughout the whole of the sintered article. By impregnating this sinter plate with nickel hydroxide, a positive accumulator electrode is obtained having especially low internal resistance.

What we claim is:

l. A process for the production of a highly porous shaped article having pores characterized by a high degree of uniformity in distribution and in width from a light metal powder obtained by thermal decomposition of a metal carbonyl and containing coarse spongy particles which comprises breaking up said particles into smaller spongy particles by forcing them through a sieve while per liter and containing coarse spongy particles which comprises breaking up said particles into smaller spongy particles by forcing them through a sieve while avoiding crushing them, and sintering the powder thus obtained.

3. A process for the production of a highly porous electrode plate for an accumulator having pores characterized by a high degree of uniformity in distribution and in width from a nickel powder obtained by thermal decomposition of nickel carbonyl and having a bulk density of about 0.5 kilogram per liter and containing coarse spongy particles which have agglomerated up to a diam eter of about 1 centimeter which comprises breaking up said particles into smaller spongy particles by forcing them through a sieve having about 400 meshes per square v 4 centimeter while avoiding crushing them, and sintering the powder thus obtained at about 800 C. in the form of an electrode plate, thereby producing an electrode plate having a high pore volume and uniformly distributed pores of about 10 to 25 microns in width.

References Cited in the file of this patent UNITED STATES PATENTS Ho'chheim' Q Dec. 29, 1931 Schulz June 1, 1937 OTHER PREFERENCES 20 pages 17-2l. 

1. A PROCESS FOR THE PRODUCTION OF A HIGHLY POROUS SHAPED ARTICLE HAVING PORES CHARACTERIZED BY A HIGH DEGREE OF UNIFORMITY IN DISTRIBUTION AND IN WIDTH FROM A LIGHT METAL POWDER OBTAINED BY THERMAL DECOMPOSITION OF A METAL CARBONYL AND CONTAINING COARSE SPONGY PARTICLES WHICH COMPRISES BREAKING UP SAID PARTICLES INTO SMALLER SPONGY PARTICLES BY FORCING THEM THROUGH A SIEVE WHILE AVOIDING CRUSHING THEM, AND SINTERING THE POWDER THUS OBTAINED. 